A vacuum pump including a motor, which is described in, for example, JP2005-184958A, is conventionally known. The above-mentioned vacuum pump is widely used to exhaust a process gas in a vacuum chamber during semiconductor manufacturing steps.
In the motor for the vacuum pump described above, a rotor chamber for separating a motor stator and a motor rotor is formed so as to seal the vacuum pump. The rotor chamber is a space which is hermetically sealed with respect to the motor stator by a partition, that is, a can fixed to the vacuum pump side. The motor having a structure in which the motor stator and the motor rotor are separated from each other by the can as described above is referred to as a canned motor. The canned motor is generally provided to an end portion of a pump rotor of the vacuum pump so as to be directly coupled to the pump rotor.
For the canned motor described above, a can made of non-magnetic metal such as stainless steel with a small thickness has been conventionally used. When the can made of non-magnetic metal is used, however, an eddy current is generated in a surface under the effect of a magnetic flux from the motor stator. By a loss generated by the generation of the eddy current, motor efficiency is lowered. On the other hand, a can member made of resin is also used. However, a thickness size of the partition is set larger as compared with that of the can member made of metal so as to maintain a mechanical strength of the partition. For example, in the case of a pump for delivering a chemical solution, in general, a pressure fluctuation is scarcely generated as a result of a reduced pressure inside the motor stator. Moreover, a pressure fluctuation in a compressing direction with respect to the can member, which is generated on the atmosphere side around the motor, is also extremely small. On the contrary, a pressure in a direction in which the can member expands outward is generally applied. In this case, the applied pressure is supported by the stator core located on the outer circumference of the can member. Therefore, even the can member having a relatively small thickness does not expand to burst. On the other hand, in the case of the canned motor for the vacuum pump, a large pressure in the compressing direction directly acts on the can member by a difference in pressure between a pressure in a vacuum region in the rotor chamber and the atmospheric pressure around the motor, which is generated during the operation. In other words, the can member is subjected to a large tensile force acting toward the interior of the rotor chamber. Therefore, in the case of the canned motor for the vacuum pump, a strength of the can member is increased by increasing the thickness of the can member to prevent radially inward buckling distortion of the can member by a compressing force.
For the canned motor including the can made of resin, when the thickness of the can is set too small, the mechanical strength of the can is lowered. As a result, there is a fear in that the can cannot resist to the pressure fluctuation in the vacuum pump. Therefore, the reduction in the thickness of the can is limited. The thickness of the can made of resin is generally set based on a pressure vessel calculating method described in JIS B8267 and the like, and is about 1.5 to 2.0 mm.